Relaxations as Key to the Magnetocapacitive Effects in the Perovskite Manganites

We present a detailed dielectric study of the relaxation effects that occur in several perovskite rare-earth manganites, including the multiferroics ${\mathrm{TbMnO}}_3$ and ${\mathrm{DyMnO}}_3$. We demonstrate that the strong magnetocapacitive effects, observed for electrical fields $\mathbf{E}\parallel \mathbf{c}$, are nearly completely governed by magnetic-state induced changes of the relaxation parameters. The multiferroic materials, which undergo a transition into a spiral magnetic state, show qualitatively different relaxation behavior than those compounds transferring into an $A$-type antiferromagnetic state. We ascribe the relaxations in both cases to the off-center motion of the manganese ions, which in the multiferroic systems also leads to the ferroelectric ordering.

Figure 1

${\epsilon }^{\prime }(T)$ of ${\mathrm{GdMnO}}_3$ at a magnetic field $\mathbf{H}\parallel \mathbf{c}$ of 1 T (a) and ${\mathrm{DyMnO}}_3$ (b) for various frequencies. The lines are guides to the eyes. The absolute values of ${\epsilon }^{\prime }$ have relatively high uncertainty due to an ill-defined geometry.

Figure 2

${\epsilon }^{\prime \prime }(\nu )$ of ${\mathrm{GdMnO}}_3$ at a magnetic field $\mathbf{H}\parallel \mathbf{c}$ of 1 T (a) and of ${\mathrm{TbMnO}}_3$ (b) for selected temperatures. The lines are fits with the Cole-Cole function. The arrow in (a) indicates the shift of the peak frequency.

Figure 3

Arrhenius plot of $\tau$ of all investigated compounds. The lines are guides to the eyes. The inset shows the results for ${\mathrm{DyMnO}}_3$, beyond the scale of the main frame.

Figure 4

Temperature dependence of $\Delta \epsilon$ (a),(b) and $\alpha$ (c),(d) of ${\mathrm{EuMnO}}_3$ and ${\mathrm{DyMnO}}_3$. The lines are guides to the eyes.